Considering the replies of Dr. Alexander and Dr. Marco I think it
is possible, in some how, to write a function which gives us directly the
underling code , something like UnderlyingCode(GAP's command); ,is

The function `ApplicableMethod' as described by Marco Costantini is exactly
such a function!
You can `Print' the result and you get the actual function.

Given the lack of scrolling capability and the missing comments it might be
more convenient to look up the code in the `lib' directory. As he described
this can be done by searching for the method installation string.